Organic light emitting devices (“OLEDs”), including both polymer and small-molecule OLEDs, are potential candidates for a great variety of virtual-and direct-view type displays, such as lap-top computers, televisions, digital watches, telephones, pagers, cellular telephones, calculators and the like. Unlike inorganic semiconductor light emitting devices, organic light emitting devices are generally simple and relatively easy and inexpensive to fabricate. Also, OLEDs readily lend themselves to applications requiring a wide variety of colors and to applications that concern large-area devices.
In general, two-dimensional OLED arrays for imaging applications are known in the art and typically include an OLED display area that contains a plurality of active pixels arranged in rows and columns. FIG. 1A is a simplified schematic representation (cross-sectional view) of an OLED structure of the prior art. The OLED structure shown includes a single active pixel 15 which includes an electrode region such as anode region 12, a light emitting region 14 over the anode region 12, and another electrode region such as cathode region 16 over the a light emitting region 14. The active pixel 15 is disposed on a substrate 10. With the aid of a sealing region 25, the cover 20 and the substrate 10 cooperate to restrict transmission of oxygen and water vapor from an outer environment to the active pixel 15.
Traditionally, light from the light emitting layer 14 was transmitted downward through the substrate 10. In such a “bottom-emitting” configuration, the substrate 10 and anode 12 are formed of transparent materials. The cathode 16 and cover 20, on the other hand, need not be transparent in this configuration.
Other OLED architectures are also known in the art, including “top-emitting” OLEDs and transparent OLEDs (or “TOLEDs”). For top-emitting OLEDs, light from the light emitting layer 14 is transmitted upward through cover 20. Hence, the substrate 10 can be formed of opaque material, while the cover 20 is transparent. In some top-emitting configurations that are based on a design like that illustrated in FIG. 1A, a transparent material is used for the cathode 16, while the anode 12 need not be transparent. In other top-emitting configurations, the positions of the anode 12 and cathode 16 in FIG. 1A are switched as illustrated in FIG. 1B, such that a transparent anode 12 is used. In this embodiment, the cathode 16 can be opaque. This is sometimes referred to as an “inverted” configuration.
For TOLEDs, in which light is emitted in both up and down directions (that is, out of both the top and bottom of the device), the substrate 10, anode 12, cathode 16 and cover 20 are all transparent. The configuration used can be like that of FIG. 1A or that of FIG. 1B.
In forming an OLED, a layer of reactive metal is typically utilized as the cathode to ensure efficient electron injection and low operating voltages. However, reactive metals and their interface with the organic material are susceptible to oxygen and moisture, which can severely limit the lifetime of the devices. Moisture and oxygen are also known to produce other deleterious effects. For example, moisture and oxygen are known to increase “dark spot areas” in connection with OLEDs. Various barrier regions are known in the art for excluding such exterior environmental species, including multi-layer structures such as those discussed in U.S. Pat. Nos. 5,757,126, 6,146,225 and 6,268,295 the entire disclosures of which are hereby incorporated by reference.
It is also known in the art to make use of distributed Bragg reflectors (DBRs) in connection with OLEDs. These DBRs are multilayer mirror structures that are typically based upon the use of a so called “quarter-wave stack”. Such structures can, among other things, narrow the spectrum of the emission band, enhance the peak radiated intensity, and modify the directionally of the emitted light. Examples of such structures can be found, for example, in U.S. Pat. No. 5,674,636 to Dodabalapur et al., U.S. Pat. No. 5,814,416 to Dodabalapur et al., U.S. Pat. No. 5,834,893 to Bulovic et al. and U.S. Pat. No. 5,920,080 to Jones. The entire disclosure of each of these patents is hereby incorporated by reference.